This button is a toggle button, just like the bold button, in that help article, toggle button are explained.

If you have selected some text, depressing this button will make
that text italic. Clicking on this button so it is not depressd will
make the text un-italisized.

If you don't select any text and you depress this button, the text
you type after this will be italisized. If you click it again before typing any
text, the next text that you type will not be italisized. If you un-depress this
button after typing some text, the text that yo uhave already typed will remain
italisized and the text that you type after this will not be italisized.

The Italics button looks like the button that is circled in the
following picture. It is not depressed in this picture.